IEEE 802.11-based wireless networks (i.e., Wi-Fi networks) have traditionally required a dedicated router or access point (AP) for operation. Newer Wi-Fi Direct devices establish peer-to-peer groups that are functionally equivalent to traditional Wi-Fi infrastructure networks. Wi-Fi Direct supports one-to-one links between a pair of devices or one-to-multiple-device links among several devices. Using Wi-Fi Direct, a device can implement either the client or the AP role depending upon how the links are established. A single device may implement both the AP and client roles in separate Wi-Fi Direct networks. The device implementing AP-like functionality in a group is referred to as the Group Owner (GO) and the other devices in the group are Clients. The GO acts as a software-enabled AP (SoftAP).
When two Wi-Fi Direct devices discover each other they negotiate their roles (Client and GO) to establish a group. Once the group is established, other clients can join the group as in a traditional Wi-Fi network. A client can establish a second group with one or more additional Wi-Fi Direct devices that act as client's in the second group. This allows for extended mesh networks of nodes that are daisy-chained together by Wi-Fi Direct devices that alternate between the Client and GO roles in different groups.
Like a traditional AP, a Wi-Fi Direct GO announces itself through beacons to establish networks or groups with one or more clients. The Wi-Fi Direct GO may also cross-connect the clients in its group to an external network (e.g., provide an interface between clients and a 3G network or an infrastructure network).
Wi-Fi Direct does not allow devices to transfer the role of GO within a group. Accordingly, if the GO leaves the group, then the group is torn down and has to be re-established with a different device (e.g., a former client) taking the role of GO.
Group formation may be established as follows.
In a standard group formation case, the Wi-Fi Direct devices discover each other and then negotiate which device will act as GO. Wi-Fi Direct devices may perform a traditional active or passive Wi-Fi scan by which they can discover existent groups and Wi-Fi networks. After this scan, a Wi-Fi Direct device executes a discovery algorithm that alternates between two states: a search state, in which the device sends Probe Requests, and a listen state, in which the device listens for Probe Requests that it responds to with Probe Responses. Once the two devices have found each other, they start a GO Negotiation phase. This is implemented using a three-way handshake, namely GO Negotiation Request/Response/Confirmation, by which the devices agree which device will act as GO and which channel the group will use for communications. Once the devices have discovered each other and agreed on the respective roles, the next phase is the establishment of a secure communication.
In an autonomous group formation case, a device may autonomously create a group in which becomes the GO by sitting on a channel and starting to beacon. Other devices discover the established group using traditional scanning mechanisms. Compared to the standard case, the discovery phase is simplified in this case because the device establishing the group does not alternate between states and no GO Negotiation phase is required.
In a persistent group formation case, Wi-Fi Direct devices can declare a group as persistent, by using a flag in a capabilities attribute in Beacon frames, Probe Responses, and GO negotiation frames. Devices forming the group store network credentials and the assigned GO and client roles for subsequent re-instantiations of the group. Specifically, after the discovery phase, if a device recognizes that a persistent group has been formed with the corresponding peer in the past, either of the devices can use an invitation procedure to quickly re-instantiate the group by reusing stored credentials.